You're more qualified than most, probably overqualified, but in this economy... it also may vary from store-to-store and area-to-area.
I hold a post-graduate degree, with a sum of a few years of experience caring for dogs and cats in several environments ranging from pet-sitting to vet-hospital. This has proven a good base of experience, but that's proven only a foundation to draw on, working at Petsmart. I don't know which position you're looking at, whether in a PetsHotel or the store-proper taking care of small animals/fish, but there is a big difference between the two.
PetsHotel appears easy to most outsiders, ("oh, so much fun to play with the doggies and kitties, sooo easy" is what I here sometimes) but it is tough, demanding work, primarily because of two factors... first, pace as set by rigorous corporate policies and procedures (very fast-paced at times, I think flipping burgers would be relaxing in comparison), and second, you are not dealing with just one dog, one mentality... you will be handling groups of dogs, with pack mentality and pack behaviours, that can quickly turn into disaster, especially in the wrong Specialist's hands. It is not easy work. Overworked and underpayed fits here IMHO.
As to small animals, it has its own challenges, and again most of those challenges come from very rigorous corporate policies and procedures and the myriad tasks you're required to keep up with every shift.
I think you're attitude and team-attitude is probably the number one thing they'll look at. I wouldn't over-emphasize you're love of animals, that certainly is a given at PetSmart, most employees are natural-born animal people. That just gets you're foot in the door. But good customer service and a focus and personal-drive toward product and pet knowledge, dependability, product recommendations and selling, these are corporate biggies.